This invention relates to a compound structure steel sheet excellent in burring workability, having a tensile strength of 540 MPa or more, and a method to produce the same, and, more specifically, to a high fatigue strength steel sheet excellent in hole expansibility (burring workability) and suitable as a material for roadwheels and other undercarriage parts of cars wherein both the hole expansibility and durability are required, and a method to produce the same.
The application of light metals such as aluminum alloys and high strength steel sheets to car components is being increased to achieve fuel economy and other related advantages through car weight reduction. Although light metals such as aluminum alloys have an advantage of high specific strength, their application is limited to special uses because of a far higher cost than steel. To further reduce car weight, therefore, a wider application of low cost, high strength steel sheets is required.
Facing the demands for higher strength, against the above background, various new steel sheets having high strength, deep drawability, bake-hardenability, etc. have so far been developed in the field of cold-rolled steel sheets used for bodies and panels, which account for a quarter or so of the total car weight, and these developments have contributed to the reduction in car weight. The focus of efforts for car weight reduction, however, has lately shifted to structural members and undercarriage components, which account for about 20% of the total car weight. In this situation, immediate action is demanded in the development of high strength hot-rolled steel sheets for these applications.
However, generally speaking, high strength is obtained at a cost of other material properties such as formability (workability) and, therefore, the key issue in the development of the high strength steel sheets is how to raise steel strength without sacrificing other material properties. Hole expansibility, fatigue resistance, corrosion resistance and the like are important among the properties required of steel sheets used especially for structural members and undercarriage components. It is essential, in this development, to realize high strength together with high values of these properties in a well-balanced manner.
Among the properties required of the steel sheets for roadwheel discs, for example, hole expansibility and fatigue resistance are regarded as particularly important. This is because burring (hole expansion) to form a hub hole is especially difficult, among various working stages, in forming a roadwheel disc and the fatigue resistance is the aspect controlled under the most stringent standards among the properties required of wheel components.
In consideration of the fatigue resistance of the wheel components, high strength hot-rolled steel sheets of 590 MPa class ferrite-martensite compound structure steel (the so-called dual-phase steel) excellent in fatigue property are presently used for the roadwheel discs. The level of strength required of the steel sheets for these components, however, is rising yet further from the 590 MPa class to the 780 MPa class. In addition to the fact that the hole expansibility tends to lower as the steel strength increases, the compound structure steel sheets are believed to be handicapped with regard to the hole expansibility because of their inhomogeneous structure. For this reason, the hole expansibility, which does not constitute any problem in the 590 MPa class compound structure steel sheets, may become a problem with 780 MPa class compound structure steel sheets.
This means that the hole expansibility is highlighted, in addition to the fatigue resistance, as an important subject in the application of high strength steel sheets to roadwheels and other undercarriage components of cars. However, despite the strong demands, few inventions have been proposed, save for a limited number of exceptions, to provide high strength steel sheets having a microstructure of a ferrite-martensite compound structure to improve the fatigue resistance, and which are also excellent in hole expansibility.
Japanese Unexamined Patent Publication No. H5-179396, for example, discloses a technology to secure the fatigue resistance of a steel sheet by forming its microstructure to consist of ferrite and martensite or retained austenite, and to ensure the hole expansibility by strengthening ferrite with precipitates of TiC, NbC, etc. so that the strength difference between ferrite grains and a martensite phase may be decreased and deformation may not concentrate locally on ferrite grains.
In the steel sheets for some of the undercarriage components such as roadwheel discs, it is essential to realize a well-balanced and high-level combination of formability such as burring workability and fatigue resistance, but the above technology does not offer these properties in a satisfactory manner. Besides, even if both the formability and fatigue resistance are satisfactory, it is important to provide a production method capable of providing these features economically and stably and, in this respect, the above conventional technology is insufficient.
To be more specific, the technology disclosed in Japanese Unexamined Patent Publication No. H5-179396 is incapable of providing a sufficient elongation because it proposes to strengthen the ferrite grains by precipitation hardening. Nor is it capable of providing a low yield ratio, which is a unique characteristic of the ferrite-martensite compound structure, because the precipitates block movable, high-density dislocations created around the martensite phase during production. Besides, the addition of Ti and Nb is not desirable since it raises production costs.
In view of the above, the object of the present invention is to provide a compound structure steel sheet capable of advantageously solving the above problems of conventional technologies, excellent in fatigue resistance and burring workability (hole expansibility) and having a tensile strength of 540 MPa or more, and a method to produce said steel sheet economically and stably.
Keeping in mind the production processes of hot-rolled and cold-rolled steel sheets presently produced on an industrial scale using generally employed steel sheet production facilities, the present inventors earnestly studied the means to achieve both good burring workability and high fatigue resistance of steel sheets. As a result, the present invention was established based on the new discovery that achieving the following was very effective for enhancing the burring workability: that microstructure is a compound structure having ferrite as the main phase and martensite or retained austenite mainly as the second phase; that the average grain size of the ferrite is 2 xcexcm or more and 20 xcexcm or less, that the quotient of the average grain size of the second phase divided by the average grain size of the ferrite is 0.05 or more and 0.8 or less, and that the carbon concentration of the second phase is 0.2% or more and 2% or less; that the quotient of the volume percentage of the second phase divided by the average grain size of the second phase is 3 or more and 12 or less; and that the quotient of the average hardness of the second phase divided by the average hardness of the ferrite is 1.5 or more and 7 or less.
The gist of the present invention, therefore, is as follows:
(1) A high fatigue strength steel sheet excellent in burring workability characterized in that:
the steel sheet is made of a steel containing, in mass,
0.01 to 0.3% of C,
0.01 to 2% of Si,
0.05 to 3% of Mn,
0.1% or less of P,
0.01% or less of S, and
0.005 to 1% or Al, and
the balance consisting of Fe and unavoidable impurities; the microstructure is a compound structure having ferrite as the main phase and martensite as the second phase; the average grain size of the ferrite is 2 xcexcm or more and 20 xcexcm or less;
the quotient of the average grain size of the second phase divided by the average grain size of the ferrite is 0.05 or more and 0.8 or less; and the carbon concentration in the second phase is 0.2% or more and 3% or less.
(2) A high fatigue strength steel sheet excellent in burring workability characterized in that:
the steel sheet is made of a steel containing, in mass,
0.01 to 0.3% of C,
0.01 to 2% of Si,
0.05 to 3% of Mn,
0.1% or less of P,
0.01% or less of S, and
0.005 to 1% or Al, and
the balance consisting of Fe and unavoidable impurities;
the microstructure is a compound structure having ferrite as the main phase and martensite as the second phase; the quotient of the volume percentage of the second phase divided by its average grain size is 3 or more and 12 or less; and
the quotient of the average hardness of the second phase divided by the average hardness of the ferrite is 1.5 or more and 7 or less.
(3) A high fatigue strength steel sheet excellent in burring workability characterized in that; the steel according to the item (1) or (2) further contains, in mass, 0.2 to 2% of Cu, and the Cu exists in the ferrite phase of the steel in the state of the precipitates of grains 2 nm or less in size consisting purely of Cu and/or in the state of solid solution.
(4) A high fatigue strength steel sheet excellent in burring workability characterized in that the steel according to any one of the items (1) to (3) further contains, in mass, 0.0002 to 0.002% of B.
(5) A high fatigue strength steel sheet excellent in burring workability characterized in that the steel according to any one of the items (1) to (4) further contains, in mass, 0.1 to 1% of Ni.
(6) A high fatigue strength steel sheet excellent in burring workability characterized in that the steel according to any one of the items (1) to (5) further contains, in mass, one or both of 0.0005 to 0.002% of Ca and 0.0005 to 0.02% of REM.
(7) A high fatigue strength steel sheet excellent in burring workability characterized in that the steel according to any one of the items (1) to (6) further contains, in mass, one or more of;
0.05 to 0.5% of Ti,
0.01 to 0.5% of Nb,
0.05 to 1% of Mo,
0.02 to 0.2% of V,
0.01 to 1% of Cr, and
0.02 to 0.2% of Zr.
(8) A high fatigue strength steel sheet excellent in burring workability characterized in that; the steel sheet is made of a steel having the chemical composition according to any one of the items (1) to (7), and the microstructure is a compound structure having ferrite as the main phase and retained austenite accounting for a volume percentage of 5% or more and 25% or less as the second phase.
(9) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by, when hot rolling a slab having the chemical composition according to any one of the items (1) to (7), completing finish hot rolling at a temperature from the Ar3 transformation temperature to 100xc2x0 C. above the Ar3 transformation temperature, holding the hot-rolled steel sheet thus produced in the temperature range from the Ar1 transformation temperature to the Ar3 transformation temperature for 1 to 20 sec., then cooling it at a cooling rate of 20xc2x0 C./sec. or higher, and coiling it at a coiling temperature of 350xc2x0 C. or lower.
(10) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by, when hot rolling a slab having the chemical composition according to any one of the items (1) to (7), applying high pressure descaling to the slab after rough rolling, completing finish hot rolling at a temperature from the Ar3 transformation temperature to 100xc2x0 C. above the Ar3 transformation temperature, holding the hot-rolled steel sheet thus produced in the temperature range from the Ar1 transformation temperature to the Ar3 transformation temperature for 1 to 20 sec., then cooling it at a cooling rate of 20xc2x0 C./sec. or higher, and coiling it at a coiling temperature of 350xc2x0 C. or lower.
(11) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by completing the hot rolling of a slab having the chemical composition according to any one of the items (1) to (7) at a temperature of the Ar3 transformation temperature or higher, subsequently pickling and cold-rolling the hot-rolled steel sheet thus produced, holding the cold-rolled steel sheet in the temperature range from the Ac1 transformation temperature to the Ac3 transformation temperature for 30 to 150 sec., and then cooling it at a cooling rate of 20xc2x0 C./sec. or higher to the temperature range of 350xc2x0 C. or lower.
(12) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by, when hot rolling a slab having the chemical composition according to any one of the items (1) to (7), completing finish hot rolling at a temperature from the Ar3 transformation temperature to 100xc2x0 C. above the Ar3 transformation temperature, holding the hot-rolled steel sheet thus produced in the temperature range from the Ar1 transformation temperature to the Ar3 transformation temperature for 1 to 20 sec., then cooling it at a cooling rate of 20xc2x0 C./sec. or higher, and coiling it at a coiling temperature of above 350xc2x0 C. and 450xc2x0 C. or lower.
(13) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by, when hot rolling a slab having the chemical composition according to any one of the items (1) to (7), applying high pressure descaling to the slab after rough rolling, completing finish hot rolling at a temperature from the Ar3 transformation temperature to 100xc2x0 C. above the Ar3 transformation temperature, holding the hot-rolled steel sheet thus produced in the temperature range from the Ar1 transformation temperature to the Ar3 transformation temperature for 1 to 20 sec., then cooling it at a cooling rate of 20xc2x0 C./sec. or higher, and coiling it at a coiling temperature of above 350xc2x0 C. and 450xc2x0 C. or lower.
(14) A method to produce a high fatigue strength steel sheet excellent in burring workability characterized by, completing the hot rolling of a slab having the chemical composition according to any one of the items (1) to (7) at a temperature of the Ar3 transformation temperature or higher, subsequently pickling and cold rolling the hot-rolled steel sheet thus produced, holding the cold-rolled steel sheet in the temperature range from the Ac1 transformation temperature to the Ac3 transformation temperature for 30 to 150 sec., then cooling it at a cooling rate of 20xc2x0 C./sec. or higher, holding it in the temperature range of above 350xc2x0 C. and 450xc2x0 C. or lower for 15 to 600 sec., and cooling it at a cooling rate of 5xc2x0 C./sec. or higher to the temperature range of 150xc2x0 C. or below.